The present invention relates to a method for heat curing of thermoset plastic materials and, in particular, to an apparatus which provides a uniformly heated and mixed thermoset plastic material for introduction into molds.
Raw thermoset plastic material is a particularly important ingredient in the manufacture of many molded plastic objects. During the manufacture of such objects, it is necessary to raise the temperature of the raw thermoset material to its cure temperature prior to its introduction into variously-shaped molds. However, one of the problems often encountered during manufacture of such articles involves control of the thermal and mechanical uniformity of the thermoset plastic mix. Homogeneous solidification is highly desirable to ensure internal structural uniformity and rugged mechanical properties for the molded article. It is, nonetheless, difficult to ensure thermal uniformity in plastic materials such as the ones contemplated for use in the present invention, because of the low thermal conductivity of raw thermoset plastic material. Accordingly, conventional methods employed to raise the barrel temperature of thermoset materials by conventional conductive means is neither energy efficient nor productive because of its low thermal conductivity.
Since the thermoset plastic material is characterized by a reduced viscosity at higher temperatures until cross-linking causes the viscosity to increase, raising the material temperature at the vicinity of the injection mold cavity is still quite desirable, however, so as to shorten the residence time of the high temperature melt in flow passages prior to its injection into various mold cavities. Under these higher temperature conditions, the melt viscosity is reduced to allow easier filling of the mold with reduced air entrapment before cross-linking causes the viscosity to increase.
In the past, one method of alleviating some of the above-mentioned problems encountered during the processing of thermoset plastic material, was the use of a divergent hot cone system. In this system, a flowing layer of plastic material is forced to flow between a heated cone and the walls of a corresponding complementarily-shaped housing wall. In this system, two modes of material heating are employed. In particular, external heaters are employed but, additionally, the material is heated frictionally by means of viscous drag forces. However, the divergent cone system introduces the thermoset plastic material at the cone tip, rather than at its base as is taught herein. Such systems as are taught herein may be generally classified as convergent hot cone systems for thermoset plastic molding processes. While divergent systems produce highly satisfactory products, improvements in homogeneity are nonetheless possible.
It is noted that if poor mixing occurs before the thermoset melt flows through the runners to the molds, the polymer melt far from the runner inlets can acquire a different "memory" of temperature and stress than the material near to the runner inlets. Therefore, melt with different memory may flow through the runners. This kind of flow situation is not desirable for molded article properties. Homogeneous solidification is a highly desirable factor for control of internal structural uniformity and rugged mechanical properties in the molded article.
Additionally, there are a wide variety of different thermoset materials which may be employed. A thermoset plastic material curing system should therefore be adaptable to process materials exhibiting a variety of temperature-viscosity profiles. In conventional divergent hot cone systems, the amount of viscous heating is predetermined by the fixed spacing between the male cone and its female counterpart in the housing itself. Accordingly, such fixed cone systems cannot accommodate a wide range of materials requiring different cure temperatures.
Furthermore, thermoset plastic materials exhibit a reduced viscosity at higher temperatures until cross-linking causes the viscosity to increase. Thus, raising the material temperature at the vicinity of the injection mold cavity is desirable to shorten the residence time of the high temperature melt in flow passages prior to injection into the mold cavity. Under these conditions, the melt viscosity reduction allows easier filling of the mold with less air entrapment prior to the occurrence of cross-linking which causes the viscosity of the material to increase. However, it is equally desirable for energy conservation reasons not to increase the melt temperature significantly beyond the cure temperature of the material being processed.
In short, it is desirable to quickly but uniformly heat large quantities of thermoset plastic material to a fixed temperature prior to injection of this material into molds. Moreover, it is particularly desirable that this heating occur uniformly throughout the thermoset melt and that it be delivered to the mold cavities in a condition of reduced viscosity. Moreover, it is desirous to have a system which is utilizable for a number of different thermoset plastic materials exhibiting a variety of cure temperatures. It is also desirable to ensure a homogeneous thermal and mechanical melt.